Optical networks employing wavelength division multiplexing (WDM) technology have come to be widely used as the demand for communication capacity has grown. WDM technology is used to multiplex and transmit a plurality of optical signals having different wavelengths.
Optical branching and insertion devices, which are also referred to as reconfigurable optical add-drop multiplexers (ROADM) and so on, are known transmission devices that employ WDM technology. Optical branching and insertion devices have optical-signal receiver/transmitters, which are also referred to as transponders and so on.
Optical branching and insertion devices multiplex optical signals of an arbitrary wavelength input from a receiver/transmitter, and transmit the multiplexed optical signals to other nodes. Furthermore, optical branching and insertion devices receive optical signals multiplexed by other nodes, separate optical signals of an arbitrary wavelength, and output the separated optical signals by way of a receiver/transmitter. In other words, optical branching and insertion devices insert and branch optical signals of an arbitrary wavelength. The wavelengths of the optical signals that are inserted and branched are set by a network management device coupled to an optical branching and insertion device.
Optical branching and insertion devices are provided with an optical cross-connect device in which an optical amplifier and a wavelength selective switch (WSS) are provided for each path so that multiplexed optical signals are transmitted between different paths. Here, a path is a transmission path extending to another node with which connection is to be made.
Wavelength selective switches select a specific wavelength from a multiplexed optical signal, and output the optical signal of the selected wavelength from an arbitrary port. Wavelength selective switches, conversely, also multiplex and output optical signals input from an arbitrary port.
Wavelength selective switches have a filter passband centered on the frequency corresponding to a selected wavelength. If a plurality of optical branching and insertion devices are coupled, the cumulative filter passband narrows, and the spectral width of the optical signal is therefore reduced. Consequently, the transmission characteristics of the optical signal deteriorate each time the optical signal passes through an optical branching and insertion device. The spectral width of the optical signal increases as the bit rate increases, in accordance with the modulation scheme of the optical signal. Consequently, for example, if a high-speed optical signal of 40 Gbps or 100 Gbps and so on is transmitted, the spectral width of the transmitted optical signal increases, the wide-range spectrum is consequently cut due to filtering at a plurality of stages, and there arises a problem in that the error rate worsens.
In relation to the narrowing of the spectrum of an optical signal, Japanese Laid-open Patent Publication No. 2010-98544, for example, discloses technology with which the filter passband of a wavelength selective switch is controlled. This technology employs a wavelength selective switch with which it is possible to seamlessly adjust the characteristics of a filter passband for allowing optical signals of each wavelength to pass.
The aforementioned technology performs band control in the wavelength selective switch in such a way that narrow passbands and wide passbands are adjacent to one another in an alternating manner. Consequently, the problem of crosstalk arises between optical signals having adjacent wavelengths. In other words, side-lobe components of the spectrum of an optical signal that is branched in a specific node propagate to adjacent nodes, and as a result a problem arises in that transmission characteristics deteriorate.
The aforementioned technology determines the passband of a wavelength selective switch in accordance with network parameters such as the number of nodes through which the optical signal of each wavelength passes. Therefore, computational processing may become complex in a management device of a network management system (NMS) or the like.